In U.S. Pat. No. 3,895,054 there is disclosed a process for the manufacture of urethanes (carbamic acid esters) by reacting an organic compound containing at least one hydroxyl group, e.g., ethyl alcohol, with carbon monoxide and a nitrogenous organic compound, e.g., 2,4-dinitrotoluene at elevated temperature and pressure in the presence of a selenium catalyst and a base and/or water.
The present invention is directed to an effective method of recovering the selenium from urethane solutions produced, for example, by the above described process and containing selenium which may be in the form of selenium per se, inorganic selenium compounds and organo selenium compounds. Because of the high reactivity of selenium, when used as a catalyst as in the above reaction its combination with organic compounds very frequently results in the formation or organoselenium compounds, such as benzoselenazole, diethyl diselenide and other soluble selenium compounds which remain in solution with the urethane reaction product. In such reaction, a portion of the selenium catalyst such as the selenium per se or selenium dioxide, selenium disulfide, etc. is converted to one or more organoselenium compounds which may be classified generally as selenols, selenides, benzoselenazoles, esters of selenocarbonic acid, selenic acid and selenious acid, selenones and the like. The type and number of organoselenium compounds which may be formed is a function of the reaction conditions used to produce the urethanes such as time, temperature, pressure and solvent. In addition, the inorganic selenium compounds used as catalysts or formed in the reaction may also remain in solution with the urethane product.
Because of the cost and toxicity of selenium, it is essential that as much of the selenium be recovered from the urethane reaction product as is possible and from the inorganic or organoselenium compounds in a form suitable for reuse as a catalyst.
Prior art processes have been proposed for the recovery of selenium from certain organic reaction products and aqueous or acidic solutions. However, such prior art processes are generally narrow in scope and application and have proven to be of little or no value to the recovery of selenium from urethane solutions containing same.
U.S. Pat. No. 3,387,928, for example, proposes dissolving a selenium-containing material (ore concentrates and solutions) in a particular aqueous acid and admixing the aqueous acidic solution with a 2-hydroxyethyl-n-alkyl ether and allowing the resulting solution to stand to effect a separation of the selenium values into an organic phase.
U.S. Pat. No. 3,577,216 discloses a process for the recovery of selenium IV used as a catalyst in the oxidative production of carboxylic acids. The selective extraction of selenious acid in the presence of an aqueous solution of inorganic salts is disclosed wherein the selenious acid may be separated by liquid-liquid extraction in counterflow with a solvent and chemical treatment. No details of the extraction or solvent are set forth.